The effects of cytochalasin B (CB) on differentiation of B cell hybridomas following stimulation with anti IgM was studied. CB did not inhibit differentiation to secretion of IgM nor the loss of membrane-bound Ia moleculares induced by anti IgM in these cells. One hybrid line was induced to secrete IgM and to reduced expression of Ia by CB alone. The results suggest that aggregation of receptors is not essential for the triggering of these cells by anti IgM. A double adoptive transfer system was used to study induction of immunologic memory in B cells specific for DNP and in T cells specific for the protein carrier KLH. T-Helper and T-suppressor activity both developed simultaneously and their levels were sustained for many months. The cells acted asynchronously following transfer: Helper cells alone together with B cells produced sustained antibody responses; suppressor cells transferred at the same time acted to terminate the antibody responses after an early peak of synthesis of antibodies.